Treatment of sewage by electrodialysis



Oct. 14, 1941. 1 MfRoBER-rs Y TREATMENT OF' SEWGE BY ELECTRODIALYSIS Filed Nov. 18, 195e Patented Oct. 14, 1941 TREATMENT F SEWAGE BY- ELECTRO- DIALYSIS l Lawrence M. Roberts, Bridgewater Township, Somerset County, N. J., assignor to Research Corporation, New York, N. Y., a corporation of New York Application November 18, 1936, Serial No. 111,499

5 Claims.

This invention relates to an improvement in the treatment of sewage-by electro-dialysis.

In the method for the electro-dialysis of sewwhich has been subjected to electro-dialysis without aeration, that produced with aeration settles more rapidly, has, a lighter color and a age, described in an application in the name of ment whereby its pI-I value is increased and a portion of its colloidal solids content is occulated and rendered separable mechanically, e. g. by sedimentation or filtration. I'he solids are separated, producing a sludge arid an eliluent, which latter ordinarily is suitable for discharge into public waterways. The sludge still contains a considerable amount of water and is otherwise objectionable as a finished product and is delivered into the anode compartment of the cell where it is subjected to electro-dialysis against the sewage in the cathode compartment. This treatment of the sludge reduces its pH value and results in dewatering the sludge, l. e. the sludge solids are coagulated further and rendered more completely separable by ltration from the liquid. The treated sludge is then subjected to mechanical separation, e. g. filtration or settling or decantation. vThe liquid or filtrate may be discharged into public waterways and the sludge solids maybe disposed of, e. g. by burning them. Said Slagle application discloses the aeration and agitation of the sewage in the cathode compartment of the electrolytic cell.

Now I have found that a suitably controlled aeration of the sludge during its treatment in the anode compartment'oil the cell aids the treatment materially. With aeration the electrodialysis o f the sludge can be carried to the desired endpoint for good filtration more uniformly and generally in less time and with less power consumption than without aeration. The aeration has special merit in the treatment of certain refractory sludges which do not readily respondl to the electro-dialysis. As compared with sludge less offensive odor, and filters to at solid residue having a higher solids content.

A result which is incidental but quite important is that the aeration increases the actual reduction in the quantity of sludge solids produced.

5 age and sludge has been arrived at, but repeated careful tests have proved that a substantial destruction of sludge solids actually results from the electro-dialysis treatment. Possible explanations of this loss are that (1) a portion of the sludge solids isrendered soluble and is eliminated with the eilluent, and (2) that organic matter is oxidizedand passes off as gaseous products.

The anodic aeration of the sludge during the electro-dialysis materially increases this lossI or destruction or elimination of sludge solids. In any event it will be appreciated that a reduction in the quantity of sludge solids produced without any apparent corresponding production of other objectionable products is a decided advantage in a large scale sewage disposal process.

Apparatus illustrating a suitable arrangement for the electro-dialysis of sewage with aeration of the sludge is illustrated in the accompanying drawing in which:

Fig. 1 is a perspective view, and

Fig. 2 is a vertical cross-section of the apparatus.

Referring to the drawing, I is a tank, 2, 2 the cathodes, 3 the anode, 4, 4 pipes for the introduction of air or other gas, and 5, 5 the diaphragms. The cathodes preferably are perforated or slotted and spaced a short distance, e. g. about .25 inch from the diaphragms. The diaphragms suitably are of asbestos iiber.y The pipes 4, 4 may be standard ceramic dilfuser tubes. y

Air or other gas is introduced` under a slight pressure sufficient to result in the formation of small bubbles on the surface of the sludge. The ow of gas is controlled so as tovavoid displacing the sludge to such an extent that the path of the material migrating to and through the diaphragm is interrupted. Uninterrupted paths for the migration of material under the iniiuence of the electro-dialysis must be maintained. To this end the gas is introduced in the form of ne streams or bubbles which diffuse up through the sludge mass and gently agitate it without s disintegratlng it as to render it a poor conductor for the electric current.

'I'he action of air introduced as described is believed to be two-fold. (1) It produces oxidation of the sludge constituents and (2) it keeps the sludge in motion at the face of the diaphragm so that all parts of the sludge are exposed to the electro-dialytic action.

The beneficial eiect of aeration is shown hy the following data from comparative tests:

` Sample 1 Sample 2 Treatment without with aeration aeration Time required .hours.; 2. 75 1. 75 Power kw. hrs/ton solids 591 469 Percent solids in sludge before electrodialysis 5. 23 23 Percentsolids in sludge after electro- 'alysis 5.02 3. 41 Percent solids in filter cake 15.7 48. 7

As appears from the foregoing data, electrodialysis .of sewage sludge without aeration reduced the percentage of solids from 5.23 to 5.02, or about 4% reduction, but with aeration the ,solids were reduced to 3.41%, or about 35% reduction. The aeration also resulted in a. shortening of the time required to bring the sludge to a satisfactory conditiony for filtration from 2.75

hours to 1.75 hours, the power consumption was to agitate the sludge without oxidation. while the air may serve to both agitate and oxidize the sludge. The results of the tests are as follows:

Sample l Sample 2 Sample 3 Nitrogen Air Nitrogen 22.3 28 21.7 Kw. hrs/ton solids..v 225 i98 267 subjecting the sewage to electro-dialysis in the cathode compartment of an electrolytic cell against sludge separated therefrom in the anode compartment through a diaphragm the step which consists in introducing a gas into the sludge in the anode compartment during the electrodialysis thereof in quantity only suiiicient to produce a mild agitation thereof adjacent the diaphragm.

2. Method of treating sewage as defined in claim 1 in'whlch the gas is inert. V

3. Method of treating sewage as defined in claim 1 in which the gas is an oxidizing gas.

4. Method of treating sewage as defined in claim 1 in which the gas is air.

5. Method of treating sewage as defined in claim 1 in which air is introduced into the sludge in ne streams adjacent the lower edge of a vertically disposed diaphragm in quantity suf.

ficient to produce mild agitation o f the sludge at least against the surface of the diaphragm but insufficient to disrupt the body of sludge.

LAWRENCE M. ROBERTS. 

